Scintillator compositions, and related processes and articles of manufacture

ABSTRACT

Scintillator materials based on certain types of halide-lanthanide matrix materials are described. In one embodiment, the matrix material contains a mixture of lanthanide halides, i.e., a solid solution of at least two of the halides, such as lanthanum chloride and lanthanum bromide. In another embodiment, the matrix material is based on lanthanum iodide alone, which must be substantially free of lanthanum oxyiodide. The scintillator materials, which can be in monocrystalline or polycrystalline form, also include an activator for the matrix material, e.g., cerium. To further improve the stopping power and the scintillating efficiency of these halide scintillators, the addition of bismuth is disclosed. Radiation detectors that use the scintillators are also described, as are related methods for detecting high-energy radiation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part and claims the benefit of theOct. 17, 2003 filing date of U.S. patent application Ser. No.10/689,361.

BACKGROUND OF THE INVENTION

In a general sense, this invention relates to materials and devices usedin the detection of ionizing radiation. More specifically, it relates toscintillator compositions which are especially useful for detectinggamma-rays and X-rays under a variety of conditions.

Many techniques are available for detecting high-energy radiation.Scintillators are of special interest, in view of their simplicity andaccuracy. Thus, scintillator crystals are widely used in detectors forgamma-rays, X-rays, cosmic rays, and particles characterized by anenergy level of greater than about 1 keV. From such crystals, it ispossible to manufacture detectors, in which the crystal is coupled witha light-detection means, i.e., a photodetector. When photons from aradionuclide source impact the crystal, the crystal emits light. Thephotodetector produces an electrical signal proportional to the numberof light pulses received, and to their intensity. Scintillator crystalsare in common use for many applications. Examples include medicalimaging equipment, e.g., positron emission tomography (PET) devices;well-logging for the oil and gas industry, and various digital imagingapplications.

As those skilled in the art understand, the composition of thescintillator is critical to the performance of the radiation detectionequipment. The scintillator must be responsive to X-ray and gamma rayexcitation. Moreover, the scintillator should possess a number ofcharacteristics which enhance radiation detection. For example, mostscintillator materials must possess high light output, short decay time,reduced afterglow, high “stopping power”, and acceptable energyresolution. (Other properties can also be very significant, depending onhow the scintillator is used, as mentioned below.)

Those skilled in the art are familiar with all of these properties. Inbrief, “light output” is the quantity of visible light emitted by thescintillator after being excited by a pulse of the x-ray or gamma ray.High light output is desirable because it enhances the radiationdetector's ability to convert the light into an electric pulse. (Thesize of the pulse usually indicates the amount of radiation energy.)

The term “decay time” refers to the time required for the intensity ofthe light emitted by the scintillator to decrease to a specifiedfraction of the light intensity at the time when the radiationexcitation ceases. For many applications, such as the PET devices,shorter decay times are preferred because they allow efficientcoincidence-counting of gamma rays. Consequently, scan times arereduced, and the device can be used more efficiently.

“Stopping power” is the ability of a material to absorb radiation, andis sometimes referred to as the material's “X-ray absorption” or “X-rayattenuation”. Stopping power is directly related to the density of thescintillator material. Scintillator materials which have high stoppingpower allow little or no radiation to pass through, and this is adistinct advantage in efficiently capturing the radiation.

The “energy resolution” of a radiation detector refers to its ability todistinguish between energy rays (e.g., gamma rays) having very similarenergy levels. Energy resolution is usually reported as a percentagevalue, after measurements are taken at a standard radiation emissionenergy for a given energy source. Lower energy resolution values arevery desirable, because they usually result in a higher qualityradiation detector.

A variety of scintillator materials which possess most or all of theseproperties have been in use over the years. For example,thallium-activated sodium iodide (NaI(Tl)) has been widely employed as ascintillator for decades. Crystals of this type are relatively large andfairly inexpensive. Moreover, NaI(Tl) crystals are characterized by avery high light output.

Examples of other common scintillator materials include bismuthgermanate (BGO), cerium-doped gadolinium orthosilicate (GSO), andcerium-doped lutetium orthosilicate (LSO). Each of these materials hassome good properties which are very suitable for certain applications.

As those familiar with scintillator technology understand, all of theconventional materials possess one or more deficiencies, along withtheir attributes. For example, thallium-activated sodium iodide is avery soft, hygroscopic material, readily absorbing oxygen and moisture.Moreover, such a material produces a large and persistent after-glow,which can interfere with the intensity-counting system. Furthermore, thedecay time of NaI(Tl), about 230 nanoseconds, is too slow for manyapplications. The thallium component may also require special handlingprocedures, in view of health and environmental issues.

BGO, on the other hand, is non-hygroscopic. However, the light yield ofthis material (15% of NaI(Tl)), is too low for many applications. Thematerial also has a slow decay time. Moreover, it has a high refractiveindex, which results in light loss due to internal reflection.

While GSO crystals are suitable for some applications, their light yieldis only about 20% of that obtained with NaI(Tl). Moreover, the crystalsare easily-cleaved. It is therefore very difficult to cut and polishthese crystals into any specific shape, without running the risk offracturing the entire crystal.

The LSO materials also exhibit some drawbacks. For example, the lutetiumelement of the crystal contains a small amount of a natural, long-decayradioactive isotope, Lu¹⁷⁶. The presence of this isotope will provide abackground count rate that can greatly interfere with highly-sensitivedetector applications. Moreover, lutetium is very expensive, and has arelatively high melting point, which can sometimes make processingdifficult.

Deficiencies of conventional scintillators have prompted the search fornew materials. Some of the new materials are described in two publishedpatent applications attributed to P. Dorenbos et al:, WO 01/60944 A2 andWO 01/60945 A2. The references describe the use of cerium-activatedlanthanide-halide compounds as scintillators. The first-mentionedpublication describes the use of Ce-activated lanthanide chloridecompounds, while the second publication describes the use ofCe-activated lanthanide bromide compounds. The halide-containingmaterials are said to simultaneously provide a combination of goodenergy resolution and fast decay constant. Such a combination ofproperties can be very advantageous for some applications. Moreover, thematerials apparently exhibit acceptable light output values.Furthermore, they are free of lutetium, and the problems sometimescaused by that element, described above. It should be noted thatlanthanide halide materials have a relatively low density relative toother rare earth halide scintillator materials. For example, lanthanunumiodide has a density of 4 g/cc to 6 g/cc. Other rare earth metal halidesmay have densities ranging from 6 g/cc to 8 g/cc.

The Dorenbos publications certainly seem to represent an advance inscintillator technology. However, such an advance is made against abackground of ever-increasing requirements for the crystals. One exampleof an end use which has rapidly become more demanding is well-logging,mentioned above. In brief, scintillator crystals (usually NaI(Tl)-based)are typically enclosed in tubes or casings, forming a crystal package.The package includes an associated photomultiplier tube, and isincorporated into a drilling tool which moves through a well bore.

The scintillation element functions by capturing radiation from thesurrounding geological formation, and converting that energy into light.The generated light is transmitted to the photo-multiplier tube. Thelight impulses are transformed into electrical impulses. Data based onthe impulses may be transmitted “up-hole” to analyzing equipment, orstored locally. It is now common practice to obtain and transmit suchdata while drilling, i.e., “measurements while drilling” (MWD).

One can readily understand that scintillator crystals used for such anapplication must be able to function at very high temperatures, as wellas under harsh shock and vibration conditions. The scintillator materialshould therefore have a maximized combination of many of the propertiesdiscussed previously, e.g., high light output and energy resolution, aswell as fast decay times. (The scintillator must also be small enough tobe enclosed in a package suitable for a very constrained space.) Thethreshold of acceptable properties has been raised considerably asdrilling is undertaken at much greater depths. For example, the abilityof conventional scintillator crystals to produce strong light outputwith high resolution can be seriously imperiled as drilling depth isincreased.

It is thus clear that new scintillator materials would be very welcomein the art, if they could satisfy the ever-increasing demands forcommercial and industrial use. The materials should exhibit excellentlight output, as well as relatively fast decay times. They should alsopossess good stopping power and good energy resolution characteristics,especially in the case of gamma rays. Moreover, the new scintillatorsshould be readily transformable into monocrystalline materials or othertransparent solid bodies. Furthermore, they should be capable of beingproduced efficiently, at reasonable cost and acceptable crystal size.The scintillators should also be compatible with a variety ofhigh-energy radiation detectors.

BRIEF DESCRIPTION OF THE INVENTION

In response to many of the needs discussed above, new scintillatormaterials have been discovered. The materials are based on certain typesof halide-lanthanide matrix materials. In one embodiment, an essentialfeature of the matrix material is that it contains a mixture oflanthanide halides, i.e., a solid solution of at least two of thehalides. The mixture usually includes a lanthanide chloride and alanthanide bromide, but can also include lanthanum iodide. Thelanthanide in the matrix is usually lanthanum itself, but can be avariety of other lanthanides. The inventors have discovered that themixture of halides results in scintillators with greatly enhancedperformance, in regard to some of the properties described above, e.g.,light output.

In another embodiment, the matrix material is based on single lanthanidehalide lanthanide halide is substantially free of LNOX, where Ln is alanthanide, and X is either chloride bromide or iodide halide, or amixture of these halides.

The scintillator materials include an activator for the matrix material.The activator can be cerium, praseodymium, or mixtures of cerium andpraseodymium. These activators provide the desired luminescence to thescintillator. Typical quantities of the activators are described below.

In some embodiments, the matrix material may further comprise bismuth.The presence of bismuth can enhance various properties, like stoppingpower. The amount of bismuth (when present) can vary to some extent andare described below.

The scintillator composition can be prepared and used in a variety offorms. The monocrystalline form is used most often. However, it issometimes desirable that the composition be in other forms as well,e.g., polycrystalline, or as a polycrystalline ceramic. Methods forpreparing the compositions in these forms (e.g., crystal growthtechniques) are also generally discussed below.

Another embodiment of the invention is directed to a radiation detectorfor detecting high-energy radiation, e.g., gamma rays. A primarycomponent of the detector is the scintillator material described above,usually in single crystal form. A photodetector (e.g., a photomultipliertube) is optically coupled to the scintillator. Since the crystalsexhibit excellent and reproducible scintillation response to gammaradiation, the detector can exhibit greatly improved performance. Thistype of radiation detector can be incorporated into a variety ofdevices, as discussed below. Two very popular applications are thewell-logging tools, and nuclear medicine tools, such as the positronemission tomography devices.

Another aspect of the invention thus relates to a method for detectinghigh-energy radiation. The method includes the use of a detector whichincorporates the unique scintillator material described herein. Methodsfor preparing such a material are also described. Some of these methodsinclude the growth of a single crystal from a molten mixture of thescintillator composition.

Further details regarding the various features of this invention arefound in the remainder of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of emission spectra for a set of scintillatorcompositions, under UV excitation.

FIG. 2 is a graph of emission spectra for a set of scintillatorcompositions, under X-ray excitation.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, the present invention includes a halide-lanthanidematrix material. In one embodiment, the matrix material is in the formof a solid solution of at least two lanthanide halides. The halides areeither bromine, chlorine, or iodine. As used herein, the term “solidsolution” refers to a mixture of the halides in solid, crystalline form,which may include a single phase, or multiple phases. (Those skilled inthe art understand that phase transitions may occur within a crystalafter it's formation, e.g., after subsequent processing steps likesintering or densification.)

The scintillator compositions of this invention are usually described interms of a matrix material component and an activator component.However, it should be understood that when the components are combined,they can be considered as a single, intimately-mixed composition, whichstill retains the attributes of activator and component. Thus, forexample, an illustrative composition in which the matrix material islanthanum bromide and the activator component is cerium bromide could beexpressed by a single chemical formula, such as (La₉₉Ce₀₁)Br₃.

The lanthanide can be any of the rare earth elements, i.e., lanthanum,cerium, praseodymium, neodymium, promethium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,and lutetium. Mixtures of two or more of the lanthanides are alsopossible. For the purpose of this disclosure, yttrium is also consideredto be a part of the lanthanide family. (Those skilled in the artunderstand that yttrium is closely associated with the rare earthgroup.) Preferred lanthanides are selected from the group consisting oflanthanum, yttrium, gadolinium, lutetium, scandium, and mixturesthereof. In especially preferred embodiments, the lanthanide islanthanum itself.

A variety of lanthanide halides can be used for this invention.Non-limiting examples include lutetium chloride, lutetium bromide,yttrium chloride, yttrium bromide, gadolinium chloride, gadoliniumbromide, praseodymium chloride, praseodymium bromide, and mixturesthereof. However, in preferred embodiments, lanthanum halides areemployed, i.e., some combination of lanthanum bromide (LaBr₃), lanthanumchloride (LaCl₃), and lanthanum iodide (LaI₃). These materials are knownin the art and commercially available, or can be prepared byconventional techniques.

In some preferred embodiments, the solid solution is based on a mixtureof lanthanum bromide and lanthanum chloride. In that instance, the ratioof the two compounds may vary considerably, i.e., a molar ratio in therange of about 1:99 to about 99:1. Very often, the molar ratio oflanthanum chloride to lanthanum bromide is in the range of about 10:90to about 90:10. In some preferred embodiments, the molar ratio is in therange of about 30:70 to about 70:30. The specific ratio of the twocompounds will depend on various factors, such as the desired propertiesmentioned above, e.g., light output and energy resolution.

The solid solution of lanthanum bromide and lanthanum chloride mayfurther include lanthanum iodide. Usually, the amount of lanthanumiodide present will range from about 0.1 mole % to about 99 mole %,based on total moles of lanthanum halide present in the solid solution.In some preferred embodiments, the amount of lanthanum iodide presentwill range from about 1 mole % to about 50 mole %. It should also beunderstood that the solid solution could comprise lanthanum iodide withjust one of lanthanum bromide or lanthanum chloride.

Continued addition of lanthanum iodide may eventually result in a phasetransition within the solid solution. It is expected that the new phasewill also have good scintillation properties. However, it may bedifficult to grow single crystals from compositions which are relativelyclose to the “cusp” of phase transition, e.g., within about 5 mole % oflanthanum iodide. Thus, those compositions are generally less preferablefor this invention, although they may be useful for some applications.(Well-known techniques can be used to determine when a phase transitionoccurs. For example, X-ray diffraction could be employed.)

In other preferred embodiments, the halide-lanthanide matrix material isbased on the use of a single lanthanide halide. For this embodiment, thelanthanide halide should be substantially free of LNOX, where LN is alanthanide and X is chlorine, bromine or iodine, or a mixture of thesehalides. As used for this particular parameter, “substantially free” ismeant to indicate a compound containing less than about 0.1 mole %oxygen, and preferably, less than about 0.01 mole % oxygen. The presentinventors have discovered that this type of oxygen-free lanthanidehalide can be used to prepare scintillators with unexpectedly improvedproperties, such as high light output.

Methods for preparing oxygen-free lanthanum halide generally follow theprocedures described below. However, in this instance, specific stepsare taken to keep oxygen completely out of the production environment.Those skilled in the art are familiar with techniques for preparingcompositions while rigorously excluding oxygen and moisture. Forexample, the starting materials can be loaded within a glove box havingan inert atmosphere, e.g., one purged with nitrogen or argon. Such anatmosphere is usually maintained at an oxygen concentration of less thanabout 100 ppm, with a moisture content of less than about 3 ppm. Theinert gasses can be purified by passage through a MnO oxygen-removalcolumn. Any solvents which are used can be distilled under argon, andstored under vacuum. Other compounds can be de-gassed and also distilledunder argon. In some laboratory or manufacturing facilities, flamedSchlenk-type glassware on a dual manifold Schlenk line is employed.High-vacuum (e.g., 10-5 torr) lines are often used. Moreover, arecirculator may be attached to the glove box. Those skilled in the artwill be able to readily determine the most appropriate procedures andequipment for a given situation.

One exemplary illustration of the preparation of lanthanum halide can beprovided. In the first step, a stoichiometric amount of ammonium iodide(NH₄IX (where X is Cl, Br, I) is combined with lanthanum oxide (La₂O₃),at about 250° C. The resulting products are an ammonium salt oflanthanumhalide, (NH₄)₃ [LaX₆], along with water and ammonia. The waterand ammonia are removed from the mixture. The ammonium salt is thenheated in vacuum at temperatures equal to or greater than about 300° C.,to form lanthanum halide(LaX₃) and a by-product, ammoniumhalide. Thelanthanum iodide can be isolated and purified. Care is taken (e.g.,excluding air and moisture), to avoid side reactions of the ammoniumsalt with lanthanum oxide, which would yield the undesired lanthanumoxyhalides (LaOX). Such materials can be made by other methods as well,under the general guidelines set forth herein. For example, lanthanumhalides can be made by reacting lanthanum carbonate or lanthanum oxidewith the corresponding halide acid, e.g., hydrochloric acid in the caseof lanthanum chloride.

As mentioned above, the scintillator composition further includes anactivator for the matrix material. (The activator is sometimes referredto as a “dopant”.) The preferred activator is selected from the groupconsisting of cerium, praseodymium, and mixtures of cerium andpraseodymium. In terms of luminescence efficiency and decay time, ceriumis often the preferred activator. It is usually employed in itstrivalent form, Ce³⁺. The activator can be supplied in various forms,e.g., halides like cerium chloride or cerium bromide.

An additional dopant may be added to increase the stopping power of thescintillator composition. Lanthanide halide scintillator compositionshave a low physical density relative to other commercially availablescintillator compositions. For example lanthanide halide compositionshave physical densities ranging from about 4 g/cc to about 5 g/cc. Otherscintillator compositions, such as BGO and LSO, have physical densitiesranging from about 7 g/cc to about 9 g/cc. In order to increase thephysical density and stopping power of lanthanide halide compositionscrystal sizes for such compositions may be larger thereby increasing thethickness of a scintillating layer on a detector. Increasing the size ofcrystals, and/or the thickness of the scintillator layer, may adverselyaffect the efficiency of the scintillating composition at least in termsof light output.

With respect to the present invention, it has been found that doping acerium activated lanthanide halide scintillator composition with bismuthwill increase the efficiency of the scintillator composition. The highatomic number of bismuth relative to other dopants may improve thestopping power of the crystals. Accordingly, bismuth may be added to thecomposition in order to increase the overall physical density andstopping power of the compostion.

The amount of activator and bismuth present will depend on variousfactors, such as the particular halide-lanthanide matrix being used; thedesired emission properties and decay time; and the type of detectiondevice into which the scintillator is being incorporated. Usually, theactivator is employed at a level in the range of about 0.1 mole % toabout 20 mole %, based on total moles of activator and halide-lanthanidematrix material. Typcially, the bismuth is employed at a level in therange of about 0.1 mole % to about 20 mole %. In many preferredembodiments, the amount of activator and bismuth is in the range ofabout 1 mole % to about 10 mole % each.

The composition of this invention may be prepared in several differentforms. In some preferred embodiments, the composition is inmonocrystalline (i.e., “single crystal”) form. Monocrystallinescintillation crystals have a greater tendency for transparency. Theyare especially useful for high-energy radiation detectors, e.g., thoseused for gamma rays.

However, the composition can be in other forms as well, depending on itsintended end use. For example, it can be in powder form. It can also beprepared in the form of a polycrystalline ceramic. It should also beunderstood that the scintillator compositions may contain small amountsof impurities, as described in the previously-referenced publications,WO 01/60944 A2 and WO 01/60945 A2 (incorporated herein by reference).These impurities usually originate with the starting materials, andtypically constitute less than about 0.1% by weight of the scintillatorcomposition. Very often, they constitute less than about 0.01% by weightof the composition. The composition may also include parasite phases,whose volume percentage is usually less than about 1%. Moreover, minoramounts of other materials may be purposefully included in thescintillator compositions, as taught in U.S. Pat. No. 6,585,913 (Lyonset al), which is incorporated herein by reference. For example,praseodymium oxide and/or terbium oxide can be added to reduceafterglow. Calcium and/or dysprosium can be added to reduce thelikelihood of radiation damage.

Methods for preparing the scintillator materials are generally known inthe art. The compositions can usually be prepared by wet or dryprocesses. (It should be understood that the scintillator compositionsmay contain a variety of reaction products of these processes). Someexemplary techniques for preparing the polycrystalline materials aredescribed in the above-mentioned Lyons patent, as well as in U.S. Pat.No. 5,213,712 (Dole), and U.S. Pat. No. 5,882,547 (Lynch et al), whichare incorporated herein by reference. Usually, a suitable powdercontaining the desired materials in the correct proportions is firstprepared, followed by such operations as calcination, die forming,sintering, and/or hot isostatic pressing. The powder can be prepared bymixing various forms of the reactants (e.g., salts, oxides, halides,oxalates, carbonates, nitrates, or mixtures thereof). Mixing can becarried out in the presence of a liquid such as water, an alcohol, or ahydrocarbon.

In one illustrative dry process, the appropriate reactants are usuallysupplied in powder form. For example, one or more lanthanide-containingreactants can be mixed with one or more halide-containing reactants, andat least one activator-containing reactant. (At least twohalide-containing reactants are used if at least two lanthanide halidesare required, as described previously). The lanthanide reactants and theactivator reactants are often oxygen-containing compounds, e.g., oxides,nitrates, acetates, oxalates, sulfates, phosphates, or combinations ofany of the foregoing. Under specified conditions, many of thesecompounds decompose to a form of the desired compounds, e.g., oxides oflanthanum and cerium. A calcining step is sometimes required to obtainthe corresponding oxides. In some preferred embodiments, the lanthanideand the halide are supplied as a single reactant, e.g., a lanthanumhalide like lanthanum chloride.

The mixing of the reactants can be carried out by any suitable meanswhich ensures thorough, uniform blending. For example, mixing can becarried out in an agate mortar and pestle. Alternatively, a blender orpulverization apparatus can be used, such as a ball mill, a bowl mill, ahammer mill, or a jet mill. The mixture can also contain variousadditives, such as fluxing compounds and binders. Depending oncompatibility and/or solubility, water, heptane, or an alcohol such asethyl alcohol can sometimes be used as a liquid vehicle during milling.Suitable milling media should be used, e.g., material that would not becontaminating to the scintillator, since such contamination could reduceits light-emitting capability.

After being blended, the mixture is fired under temperature and timeconditions sufficient to convert the mixture into a solid solution.These conditions will depend in part on the specific type of matrixmaterial and activator being used. Usually, firing will be carried outin a furnace, at a temperature in the range of about 500° C. to about1100° C. A preferred range is about 600° C. to about 800° C. The firingtime will typically range from about 15 minutes to about 10 hours.

Firing may be carried out in an oxygen-containing, inert or reducingatmosphere. Examples include air, oxygen, or a mixture of oxygen and aninert gas, such as nitrogen, helium, neon, argon, krypton, and xenon;or, one of these listed inert gases or a combination of two or moreinert gases. However, in some preferred embodiments (e.g., when thehalide is oxygen-free lanthanum iodide), firing is carried out in anoxygen-free atmosphere, as described above. After firing is complete,the resulting material can be pulverized, to put the scintillator intopowder form. Conventional techniques can then be used to process thepowder into radiation detector elements.

Methods for making the single crystal materials are also well-known inthe art. A non-limiting, exemplary reference is “Luminescent Materials”,by G. Blasse et al, Springer-Verlag (1994). Usually, the appropriatereactants are melted at a temperature sufficient to form a congruent,molten composition. The melting temperature will depend on the identityof the reactants themselves, but is usually in the range of about 650°C. to about 1050° C. In the case of lanthanum halides with acerium-based activator, the melting temperature is typically in therange of about 750° C. to about 950° C.

In most embodiments where a single crystal is desired, the crystal isformed from the molten composition by a suitable technique. A variety oftechniques can be employed. They are described in many references, suchas U.S. Pat. No. 6,437,336 (Pauwels et al); “Crystal Growth Processes”,by J. C. Brice, Blackie & Son Ltd (1986); and the “EncyclopediaAmericana”, Volume 8, Grolier Incorporated (1981), pages 286-293. Thesedescriptions are incorporated herein by reference. Non-limiting examplesof the crystal-growing techniques are the Bridgman-Stockbarger method;the Czochralski method, the zone-melting method (or “floating zone”method), and the temperature gradient method. Those skilled in the artare familiar with the necessary details regarding each of theseprocesses.

One non-limiting illustration can be provided for producing ascintillator in single crystal form, based in part on the teachings ofthe Lyons et al. patent mentioned above. In this method, a seed crystalof the desired composition (described above) is introduced into asaturated solution. The solution is contained in a suitable crucible,and contains appropriate precursors for the scintillator material. Thenew crystalline material is allowed to grow and add to the singlecrystal, using one of the growing techniques mentioned above. The sizeof the crystal will depend in part on its desired end use, e.g., thetype of radiation detector in which it will be incorporated.

Methods for preparing the scintillator material in other forms are alsoknown in the art. For example, in the case of the polycrystallineceramic form mentioned above, the scintillator material is firstproduced in powder form (or converted to powder form), as describedpreviously. The material is then sintered to transparency byconventional techniques (e.g., in a furnace), at a temperature which istypically about 65% to 85% of the melting point of the powder. Thesintering can be carried out under atmospheric conditions, or underpressure.

Yet another embodiment of the invention is directed to a method fordetecting high-energy radiation with a scintillation detector. Thedetector includes one or more crystals, formed from the scintillatorcomposition described herein. Scintillation detectors are well-known inthe art, and need not be described in detail here. Several references(of many) which discuss such devices are U.S. Pat. Nos. 6,585,913 and6,437,336, mentioned above, and U.S. Pat. No. 6,624,420 (Chai et al),which is also incorporated herein by reference. In general, thescintillator crystals in these devices receive radiation from a sourcebeing investigated, and produce photons which are characteristic of theradiation. The photons are detected with some type of photodetector.(The photodetector is connected to the scintillator crystal byconventional electronic and mechanical attachment systems).

As mentioned above, the photodetector can be a variety of devices, allwell-known in the art. Non-limiting examples include photomultipliertubes, photodiodes, CCD sensors, and image intensifiers. Choice of aparticular photodetector will depend in part on the type of radiationdetector being fabricated, and on its intended use.

The radiation detectors themselves, which include the scintillator andthe photodetector, can be connected to a variety of tools and devices,as mentioned previously. Non-limiting examples include well-loggingtools and nuclear medicine devices (e.g., PET). The radiation detectorsmay also be connected to digital imaging equipment, e.g., pixilated flatpanel devices. Moreover, the scintillator may serve as a component of ascreen scintillator. For example, powdered scintillator material couldbe formed into a relatively flat plate which is attached to a film,e.g., photographic film. High energy radiation, e.g., X-rays,originating from some source, would contact the scintillator and beconverted into light photons which are developed on the film.

Several of the preferred end use applications should also be brieflydiscussed. Well-logging devices were mentioned previously, and representan important application for these radiation detectors. The technologyfor operably connecting the radiation detector to a well-logging tube iswell-known in the art. The general concepts are described in U.S. Pat.No. 5,869,836 (Linden et al), which is incorporated herein by reference.The crystal package containing the scintillator usually includes anoptical window at one end of the enclosure-casing. The window permitsradiation-induced scintillation light to pass out of the crystal packagefor measurement by the light-sensing device (e.g., the photomultipliertube), which is coupled to the package. The light-sensing deviceconverts the light photons emitted from the crystal into electricalpulses that are shaped and digitized by the associated electronics. Bythis general process, gamma rays can be detected, which in turn providesan analysis of the rock strata surrounding the drilling bore holes.

Medical imaging equipment, such as the PET devices mentioned above,represent another important application for these radiation detectors.The technology for operably connecting the radiation detector(containing the scintillator) to a PET device is also well-known in theart. The general concepts are described in many references, such as U.S.Pat. No. 6,624,422 (Williams et al), incorporated herein by reference.In brief, a radiopharmaceutical is usually injected into a patient, andbecomes concentrated within an organ of interest. Radionuclides from thecompound decay and emit positrons. When the positrons encounterelectrons, they are annihilated and converted into photons, or gammarays. The PET scanner can locate these “annihilations” in threedimensions, and thereby reconstruct the shape of the organ of interestfor observation. The detector modules in the scanner usually include anumber of “detector blocks”, along with the associated circuitry. Eachdetector block may contain an array of the scintillator crystals, in aspecified arrangement, along with photomultiplier tubes.

In both the well-logging and PET technologies, the light output of thescintillator is critical. The present invention provides scintillatormaterials which can provide the desired light output for demandingapplications of the technologies. Moreover, the crystals cansimultaneously exhibit the other important properties noted above, e.g.,fast decay time, reduced afterglow, high “stopping power”, andacceptable energy resolution. Furthermore, the scintillator materialscan be manufactured economically, and can also be employed in a varietyof other devices which require radiation detection.

EXAMPLE 1

The example which follows is merely illustrative, and should not beconstrued to be any sort of limitation on the scope of the claimedinvention.

Six scintillator samples were examined for light output analysis. SampleA was lanthanum bromide (LaBr₃), obtained from a commercial source.Sample B was lanthanum chloride (LaCl₃), obtained in the same manner.Each of these samples served as controls.

Sample C was a composition within the scope of the present invention.The composition was a cerium-activated solid solution of lanthanumchloride and lanthanum bromide. The composition was prepared by drymixing cerium chloride with lanthanum chloride and lanthanum bromide.(All materials were commercially-obtained.) Mixing was carried out in anagate mortar and pestle. The uniform mixture was then transferred to analuminum crucible, and fired at a temperature of about 600° C. Theheating atmosphere was a mixture of 0.5% hydrogen and 99.5% nitrogen.The final molar ratio of lanthanum chloride to lanthanum bromide was66:34. (Starting material levels were adjusted to maintain the desiredproportion of halides.)

Sample D was another composition within the scope of the presentinvention. The sample was prepared in the same manner as sample C,although cerium bromide was used as the activator, rather than ceriumchloride. In this instance, the final molar ratio of lanthanum chlorideto lanthanum bromide was 34:66.

Sample E was substantially identical to sample C, but cerium bromide wasemployed as the activator, rather than cerium chloride. Sample F wassubstantially identical to sample D, but cerium chloride was employed asthe activator, rather than cerium bromide. Samples E and F were alsowithin the scope of the claimed invention.

Table 1 shows the observed light output for each scintillator material,in relative percent. The selected standard is comparative sample A, witha light output of 100%. TABLE 1 SAMPLE COMPOSITION ACTIVATOR LIGHTOUTPUT* A** LaBr₃ — 100 B** LaCl₃ — 68 C La(Cl_(0.66)Br_(0.34))₃ CeCl₃132 D La(Cl_(0.34)Br_(0.66))₃ CeBr₃ 126 E La(Cl_(0.66)Br_(0.34))₃ CeBr₃120 F La(Cl_(0.34)Br_(0.66))₃ CeCl₃ 138*Relative percent for samples B-F, as compared to sample A.**Comparative samples.

The data of Table 1 are also depicted graphically in FIG. 1, which is aplot of wavelength (nm) as a function of intensity (arbitrary units).The excitation wavelength was about 300 nm. The data show that each ofthe samples which were based on the solid solution (C, D, E, F) hadlight output values which were much higher than those of either samplesA or B. The improvement in light output was apparent in differentproportions of each halide. The improvement was also generallymaintained when the different cerium activator compounds were utilized.

EXAMPLE 2

The example which follows is merely illustrative, and should not beconstrued to be any sort of limitation on the scope of the claimedinvention.

Five scintillator samples were examined for light output analysis.Sample A was lanthanum bromide (LaBr₃), obtained from a commercialsource. This sample served as a control. It is noted that while thecontrol sample A, and the below described samples B, C, D and E did notinclude other lanthanide halides, or activators other than cerium, theinvention encompasses any of the above described scintillatorcompositions with the addition of bismuth or other elements or compoundsthat may enhance the efficiency of the scintillator composition.

Sample B was a composition within the scope of the present invention.The composition was a cerium-activated and bismuth-doped solid solutionof lanthanum bromide. The composition was prepared by dry mixing ceriumbromide and bismuth bromide with lanthanum bromide. (All materials werecommercially obtained.) Mixing was carried out in an agate mortar andpestle. The uniform mixture was then transferred to silver tube whichwas subsequently sealed, and fired at a temperature of about 800° C. Theheating atmosphere was pure nitrogen. The final molar ratio of lanthanumbromide to cerium bromide and bismuth bromide was 98:1:1. (Startingmaterial levels were adjusted to maintain the desired proportion ofhalides.)

Sample C, D and E were compositions within the scope of the presentinvention, and included the same three above-identified compoundslanthanum bromide, cerium bromide as an activator and bismuth bromide atdifferent molar ratios. The samples were prepared in the same manner assample B. In the instance of Sample C, the final molar ratio oflanthanum bromide to cerium bromide and bismuth bromide was 96:1:3.Sample D contained a final molar ratio of lanthanum bromide to ceriumbromide and bismuth bromide was 94:1:5. Sample E contained a final molarratio of lanthanum bromide to cerium bromide and bismuth bromide was89:1:10.

Table 2 shows the observed light output for each scintillator material,in relative percent. The selected standard is comparative sample A, witha light output of 100%. TABLE 2 SAMPLE COMPOSITION LIGHT OUTPUT* A**(La₉₉Ce₀₁)Br₃ 100 B (La₉₉Ce₀₁Bi₀₁)Br₃ 117 C (La₉₆Ce₀₁Bi₀₃)Br₃ 168 D(La₉₄Ce₀₁Bi₀₅)Br₃ 154 E (La₈₉Ce₀₁Bi₁₀)Br₃ 221*Relative percent for samples B-E, as compared to sample A.**Comparative samples.

The data of Table 2 is also depicted graphically in FIG. 2, which is aplot of wavelength (nm) as a function of intensity (arbitrary units).The samples were excited by X-rays. The data show that each samples B,C, D and E that were doped with bismuth had light output values whichwere comparable to or in most cases higher than than the standardcontrol that did not include bismuth. The data also shows that increasedamounts of bismuth in the cerium activated scintillator compositionresulted in higher light output values. The improvement, or at leastmaintaining a comparable light output, was also generally maintainedwhen the different halide compounds were utilized.

With respect to the present invention, the addition of bismuth to thescintallor matrix should increase the overall physical density of thescintillator. By increasing the density of the composition the stoppingpower, or ability to absorb radiation, may be improved. Having a higherphysical density also translates into utilizing smaller crystals in thescintillating matrix, which translates into reducing the thickness of ascintillating layer or matrix on a detection device. Accordingly, thebismuth may improve the scintillating efficiency of the scintillatorcomposition.

This invention has been described according to specific embodiments andexamples. However, various modifications, adaptations, and alternativesmay occur to one skilled in the art without departing from the spiritand scope of the claimed inventive concept. All of the patents,articles, and texts which are mentioned above are incorporated herein byreference.

1. A scintillator composition, comprising the following, and reactionproducts thereof: (a) a matrix material comprising one or morelanthanide halides; (b) an activator for the matrix material, comprisingan element selected from the group consisting of cerium, praseodymium,and mixtures of cerium and praseodymium; and, (c) bismuth.
 2. Thescintillator composition of claim 1 wherein the molar ratio of thelanthanide halide, the activator and bismuth is in the range of about98:1:1 to about 1:1:98.
 3. The scintillator composition of claim 2wherein the molar ratio of the lanthanide halide, the activator andbismuth is in the range of about 98:1:1 to about 89:1:10.
 4. Thescintillator composition of claim 1 wherein the molar ratio of thelanthanide halide, the activator and bismuth is in the range of about98:1:1 to about 1:98:1.
 5. The scintillator composition of claim 1wherein the molar ratio of the lanthanide halide, the activator andbismuth is in the range of about 98:1:1 to about 1:89:10.
 6. Thescintillator composition of claim 1 wherein the halide-lanthanide matrixmaterial is selected from the group consisting of (i) a solid solutionof at least two lanthanide halides, and (ii) lanthanum iodide,substantially free of oxygen.
 7. The scintillator composition of claim1, wherein the halide in the matrix material is selected from the groupconsisting of bromine, chlorine, iodine, and combinations thereof. 8.The scintillator composition of claim 1, wherein the lanthanide in thematrix material is selected from the group consisting of lanthanum,yttrium, gadolinium, lutetium, scandium, and mixtures thereof.
 9. Thescintillator composition of claim 1, wherein the lanthanide in thematrix material is lanthanum.
 10. The scintillator composition of claim6, wherein the lanthanide halides of component (i) are selected from thegroup consisting of lanthanum bromide, lanthanum chloride, lanthanumiodide, lutetium chloride, lutetium bromide, yttrium chloride, yttriumbromide, gadolinium chloride, gadolinium bromide, praseodymium chloride,praseodymium bromide, and mixtures thereof.
 11. The scintillatorcomposition of claim 6, wherein the solid solution comprises lanthanumchloride and lanthanum bromide.
 12. The scintillator composition ofclaim 11, wherein the molar ratio of lanthanum chloride to lanthanumbromide is in the range of about 1:99 to about 99:1.
 13. Thescintillator composition of claim 12, wherein the molar ratio oflanthanum chloride to lanthanum bromide is in the range of about 10:90to about 90:10.
 14. The scintillator composition of claim 6, wherein thesolid solution further comprises lanthanum iodide.
 15. The scintillatorcomposition of claim 14, wherein the amount of lanthanum iodide presentis in the range of about 0.1 mole % to about 99 mole %, based on totalmoles of lanthanide halide present in the solid solution.
 16. Thescintillator composition of claim 15, wherein the amount of lanthanumiodide present is in the range of about 1 mole % to about 50 mole %. 17.The scintillator composition of claim 6, wherein the solid solutioncomprises lanthanum iodide and one of lanthanum chloride or lanthanumbromide.
 18. The scintillator composition of claim 1, wherein theactivator is present at a level in the range of about 0.1 mole % toabout 20 mole %, based on total moles of activator and halide-lanthanidematrix material.
 19. The scintillator composition of claim 18, whereinthe activator is present at a level in the range of about 1 mole % toabout 10 mole %.
 20. The scintillator composition of claim 18, whereinthe activator comprises cerium.
 21. The scintillator composition ofclaim 1, in substantially monocrystalline form.
 22. The scintillatorcomposition of claim 1, in polycrystalline form.
 23. The scintillatorcomposition of claim 1, in the form of a polycrystalline ceramicmaterial.
 24. The scintillator composition of claim 1, in the form of afilm.
 25. The scintillator composition of claim 1, wherein the bismuthis present at a level in the range of about 0.1 mole % to about 20 mole%, based on total moles of bismuth and the halide-lanthanide matrixmaterial.
 26. The scintillator composition of claim 25, wherein thebismuth is present at a level in the range of about 1 mole % to about 10mole %.
 27. The scintillator composition of claim 25, wherein theactivator comprises cerium.
 28. A cerium-doped and bismuth-dopedscintillator composition, comprising a mixture of at least twolanthanide halides.
 29. The scintillator composition of claim 28,wherein the lanthanide halide mixture comprises lanthanum chloride andlanthanum bromide, in a molar ratio in the range of about 10:90 to about90:10.
 30. A cerium-doped and bismuth-doped scintillator composition,comprising lanthanum iodide, substantially fee of lanthanum oxyiodide.31. The scintillator composition of claim 30, wherein cerium is presentat a level in the range of about 1 mole % to about 10 mole %, based ontotal moles of cerium and lanthanum iodide.
 32. The scintillatorcomposition of claim 30, wherein bismuth is present at a level in therange of about 1 mole % to about 10 mole %, based on total moles ofcerium and lanthanum iodide.
 33. A radiation detector for detectinghigh-energy radiation, comprising: (a) a crystal scintillator whichcomprises the following composition, and any reaction products thereof:(i) a halide-lanthanide matrix material; (ii) an activator for thematrix material, comprising an element selected from the groupconsisting of cerium, praseodymium, and mixtures of cerium andpraseodymium; (iii) bismuth; and (b) a photodetector optically coupledto the scintillator, so as to be capable of producing an electricalsignal in response to the emission of a light pulse produced by thescintillator.
 34. The radiation detector of claim 33, wherein thelanthanide in the matrix material of the scintillator is lanthanum. 35.The radiation detector of claim 33 wherein the halide-lanthanide matrixmaterial is selected from the group consisting of (i) a solid solutionof at least two lanthanide halides, and (ii) a lanthanide halide that issubstantially free of oxygen.
 36. The radiation detector of claim 33,wherein the solid solution of the matrix material of the scintillatorcomprises lanthanum chloride and lanthanum bromide.
 37. The radiationdetector of claim 36, wherein the solid solution further compriseslanthanum iodide.
 38. The radiation detector of claim 33, wherein thephotodetector is at least one device selected from the group consistingof a photomultiplier tube, a photodiode, a CCD sensor, and an imageintensifier.
 39. The radiation detector of claim 33, operably connectedto a well-logging tool.
 40. The radiation detector of claim 33, operablyconnected to a nuclear medicine apparatus.
 41. The radiation detector ofclaim 39, wherein the nuclear medicine apparatus comprises a positronemission tomography (PET) device.
 42. The radiation detector of claim33, operably connected to a digital imaging device.
 43. The radiationdetector of claim 33, operably connected to a screen scintillator.
 44. Amethod for detecting high-energy radiation with a scintillationdetector, comprising the steps of: (a) receiving radiation by anactivated, halide-lanthanide-based scintillator crystal, so as toproduce photons which are characteristic of the radiation; and (b)detecting the photons with a photon detector coupled to the scintillatorcrystal; (i) a halide-lanthanide matrix material; (ii) an activator forthe matrix material, comprising an element selected from the groupconsisting of cerium, praseodymium, and mixtures of cerium andpraseodymium; and, (iii) bismuth.
 45. A method for detecting high-energyradiation of claim 45 wherein the halide-lanthanide matrix material isselected from the group consisting of (i) a solid solution of at leasttwo lanthanide halides, and (ii) a lanthanide halide that issubstantially free of oxygen.